To meet the ever-increasing capacity demand due to popular multimedia applications in mobile devices, many approaches have been proposed to enhance the system throughput for modern wireless systems. One of the most promising approaches is full duplex radio, since it transmits and receives signals using the same frequency band at the same time.
This approach can help to reduce the bandwidth footprint of the transmission while keeping the overall resulting transmission rate untouched. By means of this approach a double throughput improvement as compared to half duplex radio is achievable. Accordingly, one of the most challenging research fronts in modern wireless communication research is the design of architectures and solutions to realize effective full duplex communications.
A full duplex radio is a device which is able to transmit and receive signals on the same channel simultaneously. The most attractive and challenging solution inside this family of radio devices is the so-called in-band single antenna full duplex implementation. In this case, the device not only transmits and receives simultaneously over the same band but also does this by means of a single antenna. This can positively impact the cost of the device, which in turn does not need two or more separate circuitries and antennas to realize the full duplex communication.
However, due to the limitations of the practical hardware architecture, a very severe problem affects in-band single antenna full duplex radios, i.e., the so-called self-interference (SI). In practice, the self-interference is a portion of the transmit signal that leaks from the transmit (TX) chain to the receive (RX) chain of the device, due to the non-ideality of the employed circulator, a three input/output component which is meant to connect the antenna to both chains. If unmanaged, the SI can compromise the performance of the radio device irreversibly. This is due to its very high power as compared to the power of the incoming signal, which reaches the antenna with very low power, due to attenuations induced by the wireless propagation. Thus, the desired signal suffers from the residual SI, and the overall throughput degrades. In fact, the resulting signal to interference plus noise ratio (SINR) quantity which measures the intensity of the desired signal over the intensity of all the possible disturbances is extremely low. In this context, a correct decoding cannot be performed unless the SI can be significantly reduced, if not canceled.
As a matter of fact, in-band single-antenna full duplex radios cannot avoid a certain signal leakage from the transmit chain to the receive chain during the transmission. This induces the presence of high levels of SI affecting the received signal. At present, this is still a very challenging problem. The research in this field is still at a very early stage.
In FIG. 1, an example of a full duplex radio unit 1 is depicted. A transmission unit 3, also referred to as transmission chain TX is connected to a circulator 5, which is connected to an antenna 6 and to a reception unit 4, which is also referred to as reception chain RX. Moreover, the transmission unit 3 and the reception unit 4 are connected to an interference cancellation unit 9, which is composed of an analog interference cancellation unit 7 and a digital interference cancellation unit 8.
The circulator 5 comprises three ports A, B and C. In particular, A is the port to which the TX chain 3 is connected, B is the port to which the RX chain 4 is connected and C is the port to which the antenna 6 is connected. In practice, the circulator 5 provides limited isolation between port A and port B, resulting in interference between the TX chain 3 and the RX chain 4. The TX radio frequency (RF) signal, i.e., x, transits through port A, which routes it towards port C to reach the antenna 6. Alternatively, the received signal, i.e., z, is passed from the antenna 6 through port C, and is routed towards port B. As a matter of fact, non-ideal circulators do not provide perfect isolation between port A and port B.
Thus, a portion of the TX signal x, i.e., f(x), with |f(x)|≤|x|, leaks from port A to port B, generating interference to the received signal. Assuming the presence of a thermal noise affecting the system, usually present in non-ideal circuits, we can express the output signal from B asy=f(x)+z+n where n represents the aforementioned thermal noise, added here for the sake of simplicity in the representation.
As previously said, the exemplary solution depicted here implements a two-step SI cancellation strategy which can remove the SI component from y. The goal is to reduce the level of SI, such that its power is lower or equal than the so-called noise-floor, which is given by the sum of all the noise sources and unwanted signals within the system, previously denoted by n. As previously discussed, the SI cancellation capabilities of such a device depend on both the transmit power of the device and the noise floor. In practice, if the transmit power is high, or the noise floor is low (or a combination of the two events occurs), then residual SI interference will appear in the RX chain.
Dynamic algorithms are able to estimate the distortions introduced by the analog circuits and model the actual SI present in the RX chain 4. Accordingly, a programmable analog cancellation circuit, here referred to as analog interference cancellation unit 7 is adopted to implement them. A digital cancellation algorithm performed by the digital interference cancellation unit 8 complements the analog one to cancel the residual SI. This approach is shown to deliver around 110 dB of overall cancellation. We conclude that when the transmit power of the full duplex radio is below a certain value, the SI signal can be cancelled completely. Conversely, the desired signal suffers from the residual SI signal.
In general, we can see this as a limit for the effectiveness of the full duplex radio. Furthermore, from the point of view of energy consumption and efficiency, directly cancelling the high strength SI signal reduces the energy efficiency of the device, due to the amount of energy that is wasted. As a consequence, two main problems can be identified. On the one hand, the above shown approach still suffers from residual SI, and has an upper bound in terms of allowed transmit power for the full duplex radio. On the other hand, at present, no full duplex radio based idea tackles the problem of the energy which is wasted in the transmission/reception process.
When the full duplex radio transmits and receives signals at the same time, if the transmit power is above the maximum level that guarantees the effectiveness of the SI cancellation, the residual SI reduces the SINR of the received signal, thus affects both the spectral and the energy efficiency. In case though the transmit power is below the maximum level that guarantees the effectiveness of the SI cancellation, the full duplex system can effectively remove the SI and achieve the expected spectral and energy efficiency. Both power and energy efficiency of the full duplex radio therefore strongly depend on the effectiveness of the SI cancellation, and thus on the transmit power.